Telephone exchanges, also known as switching offices or simply switches, are connected to each other by means of trunks. These trunks (known in the art as a "trunk group") may connect a switch to another switch, to a tandem switch or to a long distance network switch. When a switch determines that a call must be routed to or through another switch, the switch sets up a trunk in the appropriate trunk group to connect the switch and the destination (or intermediary) switch. In the art, the selection process is known as "hunt". A switch "seizes" a free trunk between itself and the destination switch. The originating switch will then signal to the destination switch that the trunk has been seized. This signaling may be on the trunk itself or through a separate signaling network. The destination switch then allocates that seized trunk to the call and extends the call, either to a destination or to another switch.
Most inter office trunks are bi-directional. That is, one portion of the trunk delivers voice or data to the destination switch and the destination switch uses the other portion of the trunk to deliver voice or data to the originating switch. A known problem in the art is that of "glare" in bi-directional trunks. Glare occurs when two switches seize the same trunk for two different telephone calls at roughly the same time. Such glare conditions cause calls to be delayed in setup or may even cause an origination to fail. There have been many attempts to alleviate the glare problem, however, the problem is still present in current switching networks.